1. Field of the Invention
The present invention relates to an RF coil system in magnetic resonance imaging apparatus.
2. Description of the Related Art
In an MR (magnetic resonance) phenomenon, atomic nuclei having a magnetic moment are placed in a static magnetic field where they absorb and emit an electromagnetic wave of a specified frequency through resonance and the resonance frequency (Lamor frequency) .nu.o of the atomic nuclei can be expressed as follows: EQU .nu.o=.gamma.Ho/2 (1)
where
.gamma.: a magnetogyric ratio inherent in types of atomic nuclei; and PA1 Ho: the intensity of the static magnetic field.
An MRI system for diagnosing an inner state of a subject through the utilization of the MR phenomenon detects an electromagnetic wave generated within the subject and processes the electromagnetic wave as an MR signal. By so doing, it is possible to noninvasively obtain a slice image of the subject representing the density of the atomic nuclei, longitudinal relaxation time T1, transverse relaxation time T2, a flow and chemical shift.
A PIN diode which is controlled by a bias control circuit is connected in series with an RF (radio frequency) coil system for MRI apparatus. If an RF pulse is transmitted to the subject with the use of the RF coil system, the PIN diode is turned ON by a DC bias (forward-bias) of the bias control circuit. If an MR signal is to be detected by another receive-only coil, the PIN diode is turned OFF by a reverse bias, thereby preventing the RF coil from coupling to the receive-only coil.
Even if the PIN diode is turned OFF, a high frequency current will flow into the bias control circuit due to the connection of the bias control circuit to the RF coil. In order to prevent such occurrence, as shown in FIG. 1, an RF coil system is provided in which parallel resonance circuits 3 and 4 are connected to the anode and cathode of the PIN diode 2 in series with an RF coil 1 such that the RF coil 1 is connected via the resonance circuits 3 and 4 to a bias control circuit 5. Values of elements in the parallel resonance circuits 3 and 4 are set to resonate with the high frequency current flowing through the RF coil 1. Since, in this case, an impedance at a resonance becomes maximum in the resonance circuit, the flow of a high frequency current through the bias control circuit 5 is prevented. Furthermore, as a DC impedance of the parallel resonance circuits 3 and 4 is approximately zero, there is almost no problem in a DC bias (forward and reverse biases) of the PIN diode 2.
However, the use of the LC parallel resonance circuits, encounters a difficulty in adjusting the resonance frequency. A coupling also occurs to a variation in the surrounding environment.
Therefore, there is a growing demand for an apparatus which can prevent occurrence of coupling in surrounding environment without using an LC resonance circuit.